Conventionally, for example, the contact shown in FIG. 13 (see Japanese Patent Application Kokai No. 2003-168510) has been known as a contact of this type.
This contact 101 comprises a soldering part 102 that extends in the forward-rearward direction (left-right direction in FIG. 13) and that is soldered to a ground pattern formed on a circuit board PCB, a pair of side wall parts 103 that rise from either end portions of the soldering parts 102 in the direction of width (direction perpendicular to the plane of the page in FIG. 13), and a spring arm 104 that extends from one of the side wall parts 103. The contact 101 is formed by stamping and forming a metal plate. The spring arm 104 comprises a tongue part 104a that is bent inward from the front end of one of the side wall parts 103, a rectilinear part 104c that extends upward at an inclination toward the rear from the tongue part 104a via a first bent part 104b, a contact protruding part 104e that is bent back toward the front from the rectilinear part 104c via a second bent part 104d and curved into an upward convex shape by protruding upward from the side wall part 103, and an extension part 104f that extends forward from the contact protruding part 104e. 
The contact protruding part 104e is contacted from above by a ground conductor 110 that is positioned to face the circuit board PCB, so that the ground conductor 110 and the ground pattern formed on the circuit board PCB are electrically connected. Furthermore, the pair of side wall parts 103 are connected by connection parts 105 and 106 at both upper end portions in the forward-rearward direction. The connection part 105 toward the front contacts the extension part 104f of the spring arm 104, so that the extension part 104f is protected. Meanwhile, the connection part 106 toward the rear contacts the second bent part 104d of the spring arm 104, so that a preload is applied to the spring arm 104 by this connection part 106 contacting the second bent part 104d. Thus, as a result of the preload being applied to the spring arm 104, a load is applied to the spring arm 104 even before the ground conductor 110 contacts the contact protruding part 104e, so that the fluctuation of the load per the amount of displacement of the spring arm 104 can be reduced.
Furthermore, the contact shown in FIGS. 14A to 14D (see Design Registration No. 1108677), for example, has also been known as another conventional example of a contact.
This contact 201 comprises a soldering part 202 that extends in the forward-rearward direction (left-right direction in FIG. 14D) and that is soldered to a ground pattern formed on a circuit board PCB1, and a spring arm 203 that extends from the rear end of the soldering part 202. The contact 201 is formed by stamping and forming a metal plate. The spring arm 203 comprises a rising part 203a that rises from the rear end of the soldering part 202, a rectilinear part 203c that extends forward by being bent back from the rising part 203a via a bent part 203b, a contact protruding part 203d that is curved into an upward convex shape by protruding upward from the tip end of the rectilinear part 203c, and an extension part 203e that extends forward from the contact protruding part 203d. 
A casing or a ground pattern formed on a separate circuit board PCB2 that is positioned to face the circuit board PCB1 contacts the contact protruding part 203d from above, so that the conductor part of the casing or the ground pattern of this separate circuit board PCB2 is electrically connected to the ground pattern formed on the circuit board PCB1. Furthermore, a pair of side wall parts 204 are formed in an upright manner toward the front on either side of the soldering part 202 in the direction of width (direction perpendicular to the plane of the page in FIG. 14D), and preload application parts 205 extend inward from the upper end portions of these side wall parts 204. The preload application parts 205 are disposed on the extension part 203e of the spring arm 203, so that a preload is applied to the spring arm 203 by this contact with the extension part 203e. 
However, the following problems have been encountered in these conventional contacts.
Specifically, in the case of the contact 101 shown in FIG. 13, since the connection part 106 that applies a preload to the spring arm 104 is located farther from the tongue part 104a (i.e., the fixed end of the spring arm 104) than the contact protruding part 104e, the distance from the fixed end to the contact protruding part 104e is smaller than the distance from the fixed end to the connection part 106. Accordingly, when the ground conductor 110 contacts the contact protruding part 104e from above and continues the displacement of the contact protruding part 104e in this state, there is a danger that the second bent part 104d located beneath the connection part 106 will contact the upper surface of the soldering part 102 while the amount of displacement of the contact protruding part 104e is insufficient, so that the amount of displacement of the contact protruding part 104e will be limited by the second bent part 104d. 
Furthermore, in the case of the contact 201 shown in FIGS. 14A to 14D as well, since the preload application parts 205 that apply a preload to the spring arm 203 are positioned farther than the contact protruding part 203d from the rising part 203a constituting the fixed end of the spring arm 203, the distance from the fixed end to the contact protruding part 203d is smaller than the distance from fixed part to the preload application parts 205. Accordingly, when the casing or the ground pattern formed on the circuit board PCB2 contacts the contact protruding part 203d from above and continues the displacement of the contact protruding part 203d in this state, there is a danger that the extension part 203e located beneath the preload application parts 205 will contact the upper surface of the soldering part 202 while the amount of displacement of the contact protruding part 203d is insufficient, so that the amount of displacement of the contact protruding part 203d will be limited by the extension part 203e. It is conceivable to set the extension part 203e at a higher position in order to prevent the amount of displacement of the contact protruding part 203d from being limited. In this case, however, the preload application parts 205 must be disposed above the extension part 203e that is positioned higher than in the case of conventional contacts. Accordingly, the difference in height between the upper end of the contact protruding part 203d and the upper surfaces of the preload application parts 205 is reduced; as a result, the amount of displacement of the contact protruding part 203d is limited.
In particular, in the electrical connection between a ground pattern formed on a circuit board and a component to be connected (a casing or a ground pattern formed on another circuit board) that is positioned to face the circuit board in the field of portable telephone equipment and the like, it is desired to use a contact in which the amount of displacement of the contact protruding part is large, while the height of the contact is low. Since the amount of displacement of the contact protruding parts 104e and 203d is limited in the contacts shown in FIGS. 13, and 14A to 14D, the use of these contacts is not preferable.
FIGS. 12A and 12B show conventional examples of an electrical connector to which is applied a method for applying a preload to the spring arms of the contacts by portions of the housing. In FIGS. 12A and 12B, an electrical connector 60 comprises a plurality of contacts 80 and a housing 70 that accommodates these contacts 80, and is designed to be surface-mounted on a circuit board (not shown in the figure). The housing 70 has a plurality of first contact accommodating cavities 71 that open in the front surface (left surface in FIG. 12B) of the housing 70, and a plurality of second contact accommodating cavities 72 that open in the rear surface of the housing 70. The first contact accommodating cavities 71 and second contact accommodating cavities 72 are designed to respectively accommodate the contacts 80.
Each of the contacts 80 comprises a flat base plate 81, a soldering part 82 that extends from one end of the base plate 81 and that is soldered to a conductor pattern formed on a circuit board, and a spring arm 83 that extends from the other end of the base plate 81. Engaging parts 84 that are press-fitted to press-fitting holes respectively formed on both side walls of the first contact accommodating cavities 71 and second contact accommodating cavities 72 are provided on both sides of the base plates 81. Furthermore, each spring arm 83 extends toward one end of the base plate 81 by being bent back from the other end via a bent part; these spring arms 83 are designed to be contacted by a component to be connected (not shown in the figure) that is positioned to face the circuit board.
Furthermore, preload application parts 73 that apply a preload to the spring arms 83 are provided on the respective molded parts of the first contact accommodating cavities 71 and second contact accommodating cavities 72. The tip ends of the spring arms 83 are bent back and positioned in contact with the undersurfaces of the preload application parts 73, so that a preload is applied to the spring arms 83.
In the electrical connector 60 constructed in this manner, the soldering parts 82 of the contacts 80 that are respectively accommodated in the first contact accommodating cavities 71 and second contact accommodating cavities 72 are connected by reflow soldering to the conductor pattern formed on the circuit board, so that the electrical connector 60 is surface-mounted on the circuit board. During the connection of these soldering parts 82 by reflow soldering, the preload application parts 73 formed on the housing 70 are deformed due to the heat during this heating, so that there are cases in which the preload for the spring arms 83 is varied. In order to avoid this, it is conceivable to increase the thickness of the preload application parts 73, thus preventing the deformation of these preload application parts. However, if the thickness of the preload application parts 73 is thus increased, the height of the housing 70 will be increased.